The present invention is directed to a monomolecular rectifying diode structure and monomolecular electronic digital logic gates and higher Boolean functions based upon the monomolecular rectifying diode structure. More particularly, the present invention directs itself to a molecular conducting wire having a plurality of primarily aromatic rings that is chemically doped to integrally form a rectifying diode embedded into the molecular conducting wire. The molecular wire consists of a plurality of substantially identical aromatic ring structures bonded or linked together. The wire is chemically doped by bonding at least one electron donating group and/or an electron withdrawing group, to respective discrete portions of the molecular wire, the two portions being separated from each other by an insulating aliphatic or semi-aliphatic bridging group. The present invention also pertains to monomolecular logic gates constructed from combinations of the aforementioned rectifying molecular wire structures. Further, the present invention relates to molecular logic structures which can be formed by combination of the aforementioned logic gates to construct a larger individual molecular structure that performs a higher digital function, as, for instance, a monomolecular electronic HALF ADDER and a monomolecular electronic FULL ADDER.
For the past forty years, electronic computers have grown more powerful as their basic sub-unit, the transistor, has shrunk. However, the laws of quantum mechanics and the limitations of fabrication techniques may soon prevent further reduction in the size of today""s conventional field effect transistors. Many researchers project that during the next 10-15 years, as the smallest features on mass-produced transistors shrink from their present approximate length range of 100 nanometers to 250 nanometers, the devices will become more difficult and costly to fabricate. In addition, they may no longer function effectively in ultra-densely integrated electronic circuits. In order to continue the miniaturization of circuit elements down to the nanometer scale (nanoelectronic), perhaps even to the molecular scale, researchers are investigating several alternatives to the solid state transistor for ultra-dense circuitry. However, unlike today""s FETs, which operate based on the movement of masses of electrons in bulk matter, the new devices take advantage of quantum mechanical phenomena that emerge at the nanometer scale.
There are two broad classes of alternative nanoelectronic switches and amplifiers:
(a) solid state quantum-effect and single electron devices, and
(b) molecular electronic devices.
Devices in both classes take advantage of the various quantum effects that begin to dominate electron dynamics on the nanometer scale. Despite the novelty of the designs of solid state quantum-effect and single electron devices, researchers already have been able to develop, fabricate, and employ in circuitry several promising new device types by building upon 50 years of industrial experience with bulk semiconductors. Such solid-state quantum-effect devices change the operating principles for ultra-miniature electronic switches, but they still bear the difficult burden of requiring that nanometer-scale structures be xe2x80x9ccarvedxe2x80x9d out of amorphous or crystalline solids.
Molecular electronics is a relatively new approach that would change both the operating principles and the materials used in electronic devices. The incentive for such radical change is that molecules are naturally occurring nanometer-scale structures. Unlike nanostructures built from bulk solids, molecules can be made identically, cheaply, and easily that will be needed for industrial scale production of ultra-dense computers. Two of the significant challenges are to devise molecular structures that act as electrical switches, and to combine these molecules into a more complex circuit structure needed for computation of applications.
Presently, there are two primary types of small molecules that have been proposed and/or demonstrated for use as molecular scale electrical conductors. These two types of molecular-scale conductors are: (a) polyphenylene-based conductors, and (b) carbon nanotubes.
Polyphenylene-based molecular wires involve chains of organic aromatic benzene rings bonded to each other, shown in FIG. 1A or linked to each other by acetylene spacers, as shown in FIG. 1B. Until recently, whether such small molecules had appreciable conductance was an open question. However, over the last two or three years, molecules of this type have been shown by several research groups to conduct small electrical currents.
An individual benzene ring, shown in FIG. 2, has the chemical formula C6H6. When a benzene ring is drawn, as shown in FIG. 3A, with one of the hydrogen atoms removed (e.g., to form C6H5), so that it can be bonded as a group to other molecular components, such a ring-like substituent group is termed a phenyl group. When two hydrogen atoms are removed from a benzene ring (e.g., C6H4), a phenylene ring is obtained, as shown in FIG. 3B. By binding phenylenes to each other on both sides of the respective rings and terminating the resulting chain-like structures with phenyl groups, a polyphenylene-based molecule is thus formed. Thus, molecules made primarily from two or more phenyl groups are known as polyphenylenes.
While polyphenylene chains do not carry as much current as carbon nanotubes, they are very conductive small molecules. Also, polyphenylenes have the great advantage of a very well-defined chemistry and great synthetic flexibility, based upon more than a century of experience accumulated by organic chemists in manipulating such aromatic compounds. Recently, James M. Tour has refined the synthetic techniques for conductive polyphenylene chains (or molecular wires) by developing precise synthetic methods that produce enormous numbers of these molecules, approximately 1023, every one of which is of exactly the same structure and length. Such polyphenylene-based molecular wires, shown in FIGS. 1A and 1B, have come to be known as Tour wires. While the Tour wires provide conductive leads, the molecular-scale electrical devices that they interconnect must have a structure and chemistry that is compatible therewith.
The source of the conductivity for a polyphenylene-based wire is the conjugated pi-type orbital that lies above and below the plane of the molecule when it is in its planar or near planar conformation, as shown in FIG. 4. In such a conformation, the pi-orbitals associated with the individual ring-like phenyl groups overlap to create a single pi-orbital that runs the length of the molecule, because of the significant energetic advantage that arises from delocalizing the pi-electrons over the length of the entire molecule. Because this long pi-orbital is both out of the plane of the nuclei in the molecule, and it is relatively diffuse compared to the in-plane sigma molecular orbitals, the pi-orbital forming a xe2x80x9cchannelxe2x80x9d or xe2x80x9cconduction bandxe2x80x9d that can permit transport of additional electrons from one end of the molecule to the other when it is under a voltage bias. In FIG. 5, such a pi-type conduction channel is sketched for a Tour wire.
In practice, as shown in FIG. 1B, triply bonded acetylenic linkages often are inserted as spacers between the phenyl rings in the Tour wire. These spacers eliminate the steric interference between hydrogen atoms bonded to adjacent rings. Otherwise, these steric interferences would force the component rings in the Tour wire to rotate into a non-planar conformation that would reduce the extent of conjugation between adjacent rings, break up the electron channel, and decrease the conductivity of the wire. The acetylenic linkages themselves, because of their own out-of-plane pi-electron density become a part of the electron channel, and thus, they permit the conductivity to be maintained throughout the length of the molecule.
On the other hand, aliphatic organic molecule, singly bonded molecules that contain only sigma bonds on the axes of the atoms bonded to form the molecule, do not form an uninterrupted channel outside the plane of the nuclei in the molecules. They have nodes or breaks in the electron density at the positions of the atomic nuclei. Therefore, such singly bonded structures cannot transport an unimpeded electron current when they are placed under a voltage bias. That is, aliphatic chains, or groups, shown in FIG. 6, act as insulators. It follows then that when a small aliphatic group is inserted into the middle of a conductive polyphenylene chain, it breaks up the conductive channel and is said to form a barrier to electron transport.
The second type of demonstrated molecule conductor is, as mentioned above, the carbon nanotube, also known as a buckminsterfullerene tube (or buckytube). This type of structure can make an extremely conductive wire. On the other hand, buckytubes, having only been discovered and characterized within the past two decades, do not have a very well-defined chemistry. Buckytubes tend to be very stable structures when formed, such that their chemical formulation and manipulation in bulk chemical processes only occurs under relatively extreme conditions. Such reactions tend not to be very selective or precise in the range of buckytube structures. The selection of buckytubes for use in electronic devices requires the use of physical inspection and manipulation of the molecules one by one, in order to segregate the molecules of choice from among many carbon nanotube molecules, some of which may have a range of different structures.
As is known, a diode is a two terminal switch, which can turn a current xe2x80x9conxe2x80x9d or xe2x80x9coffxe2x80x9d. Two types of molecular-scale electronic diodes have been demonstrated recently: (a) rectifying diodes, and (b) resonant tunneling diodes.
A molecular rectifying diode, or molecular rectifier, functions by making it more difficult to induce electrons to pass through it in one direction, usually termed the xe2x80x9creversexe2x80x9d direction, then in the opposite xe2x80x9cforwardxe2x80x9d direction. Molecular rectifiers were first disclosed in the first scientific paper on the subject of molecular electronics, by Aviram and Ratner, in their 1974 work entitled xe2x80x9cMolecular Rectifiersxe2x80x9d. Aviram and Ratner based their suggestions for the structure of a molecular diode switch on the operational principles of the solid state, bulk-effect pn-junction diode which were well-known at the time. According to Aviram and Ratner, their molecular rectifier consists of a donor pi-system and an acceptor pi-system, separated by a sigma-bonded insulated bridge. The hemiquinone molecules shown in FIG. 7, were suggested as a prototype of a molecular rectifier. The quino (=0) groups on the left decrease the pi-density and raise the electron affinity, whereas the methoxy (xe2x80x94OCH3) groups on the right increase pi-density and lower ionization potential. These two parts of the molecular rectifier are separated and insulated from each other by a dimethylene (xe2x80x94(CH2)2xe2x80x94) bridge, which is a sigma bridge. After nearly 25 years, the rectifying diode behavior of such molecular structures has been recently demonstrated. However, a device thus formed, as with quinones, is not readily bonded with polyphenylene conducting wires, and any such joining results in a great loss of conductivity.
In view of extremely high suitability of Tour wires for molecular electronic structures, several other types of non-rectifying molecular diodes have been devised based on polyphenylene wires as the xe2x80x9cbackbonexe2x80x9d. For instance, a molecular resonant tunneling diode (RTD) has been developed, which takes advantage of energy quantization in a manner that permits the amount of voltage bias across the source and drain contacts of the diode to switch xe2x80x9conxe2x80x9d and xe2x80x9coffxe2x80x9d and electron current travelling from the source to the drain. Depicted in FIG. 8A, is a molecular resonant tunneling diode (RTD) that has been synthesized by James M. Tour and demonstrated by Mark A. Reed in 1997. Structurally and functionally, the device is a molecular analog of the much larger solid state RTDs that for the past decade have been fabricated in quantity in III-V semiconductors. Based upon a Tour wire backbone, as shown in FIG. 8A, Reed""s and Tour""s polyphenylene molecular RTDs are made by inserting two aliphatic methylene groups into the wire on either side of a single benzine ring. Because of the insulating properties of the aliphatic groups, as discussed in previous paragraphs, they act as potential energy barriers to current flow. They define the benzine ring between them as a narrow, approximately 0.5 nm, xe2x80x9cislandxe2x80x9d of lower potential energy through which electrons must pass in order to traverse the length of the molecular wire.
Whenever electrons are confined between two such closely spaced barriers, quantum mechanics restricts their energies to one of a finite number of discrete quantized levels. This energy quantization is the basis for the operation of the resonant tunneling diode. The only way for electrons to moderate kinetic energies to pass through the device is to tunnel, quantum mechanically, through the two barriers surrounding the xe2x80x9cislandxe2x80x9d. The probability that the electrons can tunnel from the source region onto the island is dependent on the energy of the incoming electrons in the source compared to the unoccupied energy levels on the island of the device. As illustrated in FIG. 8B, if the bias across the molecule produces incoming electrons with kinetic energy, that differ from the unoccupied energy level available inside the potential well on the island, the current does not flow. The RTD is switched xe2x80x9coffxe2x80x9d.
However, if the bias voltage is adjusted so that the kinetic energy of the incoming electrons aligns with that of one of the internal energy levels, as shown in FIG. 8C, the energy of the electrons outside the well is said to be in resonance with the allowed energy inside the well. Then, current flows through the device, i.e., the RTD is switched xe2x80x9conxe2x80x9d.
Although using a polyphenylene wire, this molecular resonant tunneling diode is not a rectifying diode, nor does it use a donor-acceptor principle of the rectifying diode.
U.S. Pat. No. 5,475,341 to Reed discloses a micro-electronic semiconductor integrated circuit device integrated on a common substrate with molecular electronic devices, having a barrier-well-barrier structure with the well being conductive oligomer. Similar to the molecular RTD discussed in the foregoing paragraphs, the molecular electronic devices, disclosed in the ""341 Patent, are not donor-acceptor complex based diodes.
The published International Application WO93/25003 is directed to a sub-nanoscale and electronic system, in which micro-electronic semiconductor integrated circuit devices are integrated on a common substrate with molecular electronic devices. Disclosed is a process of a self-aligning of molecular wires to their target terminals. Polymeric molecular structures are manipulated to produce combinations of well and barrier regions with connections, so that the well and/or barrier potential can be manipulated. Thus, the structures disclosed in the WO93/25003 connect to the molecular wires, as opposed to being formed therein, nor does the reference disclose donor-acceptor principles as a basis for a rectifying diode.
U.S. Pat. No. 3,953,874 is directed to an organic electronic rectifying device. In this system, hemiquinone is utilized to form asymmetric donor acceptor structures. However, the reference does not suggest use of polyphenylene conductors into which a molecular rectifying diode is formed.
Despite the benefits of polyphenylene conducting wires in nanometer-scale, molecular electronics, there was no suggestion in the prior art of using Tour wires as the conductive backbone for rectifying diodes. Nor has the prior art suggested chemical modification of molecular conductive wires for implementing donor-acceptor principles to embed rectifying diodes in the wire itself. Further, there is not a teaching or suggestion in the prior art of conductive molecular electronic digital logic structures i.e., AND, OR, and XOR gates, based on molecular rectifying diodes. Nor has there been disclosed or suggested such complex conductive molecular logic circuits as a molecular HALF ADDER or molecular FULL ADDER, using a polyaromatic conductive wire backbone.
It is therefore an object of the present invention to provide a monomolecular rectifying diode structure formed by selective doping of a molecular conducting wire having a plurality of primarily aromatic rings. The selective doping forming one or more respective donor and/or electron acceptor sites.
A further object of the invention is to provide electrically conductive monomolecular digital logic structures. The monomolecular digital logic structures include such Boolean functions as a monomolecular AND gate, a monomolecular OR gate, a monomolecular XOR gate, a monomolecular HALF ADDER, and a monomolecular FULL ADDER. The monomolecular logic structures are formed from monomolecular rectifying diode structures and operate by the conduction of electrical current with a bias voltage applied to the molecular circuit.
In accordance with the teachings of the present invention, a monomolecular electronic device is provided. The device comprises at least one molecular conducting wire having a plurality of joined substantially identical aromatic ring structures. A molecular insulating group is bonded between a respective pair of the aromatic ring structures to establish two sections of the conducting wire. The monomolecular electronic device further includes a doping structure for at least one of the two sections of the conducting wire, to form at least one of an electron donor site and an electron acceptor site. That combination integrally forms a current carrying rectifying diode in the molecular conducting wire, the combination being a single molecule.
Viewing the present invention from another aspect, the present invention is directed to a monomolecular electronic rectifying device that includes a polyphenylene-based conducting wire having a plurality of joined molecular ring structures. The rectifying device further includes a molecule of an insulating group bonded between a respective pair of the molecular ring structures to establish two sections of the conducting wire. A first dopant bonded to a respective one of the two sections is included to form a respective electron donor site. Additionally, a second dopant bonded to the other of the two sections is also included to form an electron acceptor site. That combination integrally forms a rectifying diode in the conducting wire.
Viewing the present invention from still another aspect, the present invention is directed to a monomolecular logic structure that includes a first molecular conducting wire having a rectifying diode embedded therein. The first molecular conducting wire has an end for receiving a fist input signal current. The logic structure also includes at least a second molecular conducting wire having a rectifying diode embedded therein. The second molecular conducting wire has an end for receiving a second input signal current. The logic structure further includes at least one node defined by an aromatic ring chemically bonded to a respective portion of both the first and second molecular conducting wires. An output molecular branch chemically bonded to the node is also provided. The output molecular branch is formed by at least one aromatic ring. A resistive molecular branch joined to the node is further provided for applying a reference potential thereto and the combination of elements thereby defining a single molecule for combining the first and second input signal currents in accordance with a predetermined Boolean logic function. The predetermined Boolean logic function may include an AND gate, an OR gate, an XOR gate, a HALF ADDER, a FULL ADDER, and combinations thereof.
From a yet further aspect, the present invention is directed to a monomolecular logic structure that includes a plurality of molecular conducting wires joined together to form a single molecule having a predetermined structure. A selected portion of the plurality of molecular conducting wires each being chemically doped to form a current carrying rectifying diode therein. The predetermined structure defining a predetermined Boolean logic function that may include an AND gate, an OR gate, an XOR gate, a HALF ADDER, a FULL ADDER, and combinations thereof.
These and other novel features and advantages of this invention will be fully understood from the following Detailed Description and the accompanying Drawings.